The present invention relates to machines and processes for forming longitudinally curved tooth gears. In particular, the invention relates to machines and processes which use a dish-shaped tool for precision forming of curved tooth gears, such as spiral bevel and hypoid gears.
For over 50 years, dish-shaped grinding wheels have been used on gear grinding machines for finish grinding spiral bevel and hypoid gears. These machines are arranged for rotating a dish-shaped grinding wheel about its axis while simultaneously oscillating the grinding wheel through an arc corresponding to the desired longitudinal shape of a bevel or hypoid gear tooth. Advantages related to the use of such dish-shaped grinding wheels are well known and clearly disclosed in U.S. Patent to Taylor, U.S. Pat. No. 1,830,971. A primary advantage of these grinding wheels is that they provide sufficient clearance between the operating surfaces of the wheels and tooth surfaces being ground to promote access of coolant to the grinding area and to resist overheating and damage to tooth surfaces.
According to known methods for grinding hypoid or bevel gear teeth, machines with dish-shaped grinding wheels are used to simultaneously form-grind adjacent tooth sides of one member of a work gear pair, but the adjacent tooth sides of the other gear member are generated separately with a conventional cup-shaped grinding wheel. The separate treatment of the gear tooth sides in one member of a work gear pair has been required to appropriately mismatch the longitudinal tooth curvatures of the form-ground member. Typically, a small amount of mismatch between mating tooth curves is desirable to permit some adjustment in the operating positions of the mating gears, however, the amount of mismatch associated with an attempt to simultaneously form adjacent gear teeth in both members of a mating gear pair by known methods, may greatly exceed desirable mismatch between mating tooth curves. Likewise, the formation of adjacent gear tooth sides of at least one member of a mating work gear pair in separate working steps or operations, as an alternative to simultaneously forming adjacent gear teeth in both members, is time consuming and may add considerable cost to the manufacture of the work gear pair.
Also according to known method, longitudinal mismatch is provided between mating bevel and hypoid gear teeth by adjusting the radius of a cup-shaped grinding wheel used to grind a tooth side on one member of a gear set with respect to the radius of oscillation of a dish-shaped grinding wheel used to grind a mating tooth side of the other member of the pair. Longitudinal mismatch between mating tooth sides is determined by the difference between the respective radii of the cup-shaped wheel and arcuate path of oscillation of the dish-shaped wheel. Thus, known mismatch between mating bevel and hypoid gear teeth may be represented as the separation between two arcs of different radii that are theoretically coincident at a single point. Under load, however, mating gear teeth tend to deform slightly and contact between the two surfaces spreads out over a portion of tooth length.
Although known mismatched bevel and hypoid tooth surfaces may accommodate some adjustments to their running positions, contact between mating tooth surfaces tends to shift quite rapidly to tooth ends under conditions of misalignment or heavy loads. Contact at tooth ends is highly undesirable and may cause excessive noise or tooth breakage. It is also known to increase mismatch by increasing the difference between the respective radii of tooth curvature of mating tooth surfaces. Increased mismatch tends to resist the shifting of tooth contact to tooth ends. However, increased mismatch also tends to undesirably shorten the contact pattern between mating gear teeth leading to increased contact stresses and wear, and decreased load carrying capacity of the gear teeth. Accordingly, the amount of adjustment which may be accommodated by mismatched bevel and hypoid gear teeth formed according to known methods may be undesirably limited by the load carrying requirements of the gear teeth.